When constructing new radio communication systems, there is a need for increasing data rates in the system in relation to data rates of preceding radio communication systems. New services are constantly provided, which require higher and higher data rates. Also, in order not to force users of the preceding radio communication system to buy new equipment, such as User Equipments (UEs), every time a new radio communication system or an updated version of an old system is launched, backwards compatibility should preferably be assured in the new or updated radio communication system. This gradual change of equipment requirements also gives the users some time to exchange their equipment.
In this document, embodiments of the invention will be exemplified for the Third Generation Partnership Project Long Term Evolution Advanced (3GPP LTE-Advanced) system, and thus for Advanced E-UTRA, for which discussions within the 3GPP have recently been started. However, the embodiments of the invention are applicable to essentially any system utilizing multicarrier transmission, such as Orthogonal Frequency Division Multiplexing (OFDM) transmission, as is clear for a person skilled in the art.
As discussed within the 3GPP, the LTE-Advanced system should build on backwards compatibility to the Third Generation Partnership Project Long Term Evolution (3GPP LTE) system, and is supposed to meet the International Telecommunication Union (ITU) requirements for the International Mobile Telecommunication Advanced (IMT-Advanced) system. In this document, the term LTE is generally used for denoting Evolved Universal Terrestrial Radio Access/Evolved Universal Terrestrial Radio Access Network (E-UTRA/E-UTRAN). Correspondingly, the term LTE-Advanced is in this document used for denoting Advanced E-UTRA/Advanced E-UTRAN. For the LTE/LTE-Advanced case, as is also specified by 3GPP, backwards compatibility means that LTE User Equipment (UE) should be able to work in the LTE-Advanced system. Correspondingly, here, and throughout in this document, “backwards compatible” (or simply “compatible”) means that equipment of a legacy system (old system) should be able to work in a new system being defined.
In general, the new system, having higher data rates, e.g. an LTE-Advanced system or the like, should provide the requirements:                backwards compatibility to a legacy system, e.g. an LTE system or the like,        low complexity for a transmitter in the new system, e.g. a low complexity LTE-Advanced transmitter, and        high spectral efficiency.        
As was stated above, backwards compatibility should assure that equipment of the legacy system, e.g. an LTE UE device, is functional also in the new system, e.g. the LTE-Advanced system. This is particularly important during the initial stages of the new system, when the fraction of equipment of the new system will be much smaller than the fraction of equipment of the legacy system. For instance, in the initial stages of LTE-Advanced, before most of the users have bought LTE-Advanced equipment, the number of LTE UEs in the system will be higher than the number of LTE-Advanced UEs.
Further, one characterizing feature of the exemplified IMT-Advanced and LTE-Advanced systems is the support of very high data rates, possibly up to 1 Gbps in downlink direction. This will undoubtedly require a very large transmission bandwidth, maybe even up to 100 MHz. For example, LTE currently supports scalable transmission bandwidths up to 20 MHz. Thus, there is a transmission bandwidth problem of providing larger transmission bandwidths for such very high data rates.
For instance, LTE uses OFDM modulation with a fixed subcarrier spacing, which is well suited to provide scalable bandwidth, simply by adding more subcarriers. The smallest time-frequency resources in LTE are denoted resource blocks, each of width 180 kHz and duration of 1 ms. The normal subcarrier spacing in LTE is 15 kHz and the carrier frequency raster is 100 kHz. However, there is also a mode with 7.5 kHz subcarrier spacing. For LTE, larger bandwidths could in principle be facilitated by increasing the number of resource blocks even more. This would however require major changes in the standard specifications, for instance regarding control signalling, as the system according to the standard is designed assuming a limit on the number of resource blocks to 110.